The present invention is generally related to an apparatus useful for introducing a sample into a graphite tube in flameless atomic absorption spectroscopy, and, in particular, relates to such an apparatus including a sample carrier of electrically conducting material adapted for insertion into the graphite tube.
In conventional flameless atomic absorption spectroscopy a sample is introduced through a lateral introduction aperture in the wall of a graphite tube, which tube is usually retained between two annular electrodes. A strong electrical current is passed through the electrodes whereby the graphite tube, and therewith the sample, are heated to a high temperature. At a predetermined temperature, the sample is atomized, to produce a "cloud of atoms" of the sought component therein in the graphite tube.
A measuring light beam of an atomic absorption spectrophotometer is passed in a longitudinal direction through the graphite tube and the annular electrodes. This measuring light beam emanates from a spectral line emitting light source, for example a hollow cathode lamp, and preferably comprises only the resonant lines of the sought element. Ideally, the measuring light beam is absorbed only by the atoms of the sought component of the measuring light beam and is thus indicative of the concentration of the sought component.
Ordinarily a sample exists in the liquid state as a solution. In order to prevent the measurement from being affected by the solvent thereof and to ensure rapid atomization for the measurement, the sample is initially dried at a temperature lower than the atomization temperature, during the drying the solvent is vaporized. This drying is usually followed by an "ashing process", wherein the sample is thermally decomposed at a temperature elevated with respect to the drying temperature but less than the atomization temperature. During the ashing process, carbon can be produced, which carbon is formed by the non-vaporized components of the sample. The presence of such carbon can falsify the measurement by absorbing the measuring light beam. Generally, these interfering components are carried away prior to the measurement proper via an inert gas flow, which gas flow continuously through the graphite tube to prevent the entrance of air thereinto which would thus burn out the graphite tube. In prior art graphite tube atomizers, the inert gas flow is admitted from the ends of a graphite tube such that it emerges through the introduction aperture.
In a prior art "graphite tube atomizers" of this type the sample is injected into the graphite tube such that it accumulates nearly in the center of the graphite tube on the lower portion of the inner wall. The temperature is then changed in accordance with a predetermined program for the drying, ashing and atomizing processes.
In such a procedure, the drying and ashing steps occur inside the graphite tube, which is a straight continuous tube, in the radiation path of the measuring light beam. Thereby a signal appears at the detector exposed to the measuring light beam. Interfering components of the drying and ashing process, which are not completely blown out of the graphite tube, can condense on the inner wall of the graphite tube and falsify the measurement.
In addition the atomization temperature, at which a sought component is atomized in the sample, often depends upon the type of compound in which the element is present in the dried and ashed sample. If, then, the graphite tube is heated continuously after the ashing, a sought component can be atomized first from one compound and subsequently, at a higher temperature, from another compound. This double temperature atomization results in corresponding signals at the detector and the unambiguity of the relation between peak height of the detector signal and quantity of the sought component is no longer a valid measurement.
For this reason it is known to apply the sample solution, as a droplet, to a carrier, for example a wire helix of tungsten wire. The sample carrier including the sample solution is then moved in front of the introduction aperture of the graphite tube. The sample is then dried by the hot inert gas flow emerging from the introduction aperture through the vaporized solvent does not enter the graphite tube, at all. Such a technique has been discussed in an article published in Analytical Chemistry, volume 51 (1979), 2375-2378.
When the sample carrier is moved closer to the graphite tube, the temperature of the sample carrier is further increased via the heat transfer from the graphite tube. In this manner the dried sample is further heated and thermally decomposed. This also takes place outside of the graphite tube. Thereafter, the graphite tube is heated to the atomization temperature. After this temperature has been reached, the sample carrier is quickly inserted into the graphite tube.
In this way, condensation of interfering components from the drying and ashing process on the inner wall of the graphite tube is prevented. The dried and decomposed sample is heated at once to the predetermined atomization temperature by inserting the sample carrier into the tube after the temperature has been reached. Hence, the atoms of the sought component form the cloud of atoms simultaneously, independent of their chemical compound.
Such an arrangement suffers from the disadvantage that the drying and ashing temperatures are dependent upon the heat transfer between the graphite tube and the sample carrier and thus are not exactly defined. For example, the graphite tube must be maintained at a higher temperature than the sample carrier in order to heat the sample carrier and the sample.
From U.S. Pat. No. 4,162,849, issued on July 31, 1979, a method for concentrating a sought component of a sample in a graphite tube for flameless atomic absorption spectroscopy is known in which the sample solution is introduced into a crucible and the crucible is heated to a first temperature at which the sought component is volatile. The graphite tube is maintained at a second temperature below the first temperature. An inert gas flow is then passed over the crucible and then through the graphite tube. The components of the sample vaporizing at the first temperature of the crucible condense on the walls of the graphite tube. The apparatus required herefor is, however, rather expensive.